1. Field of the Invention
This invention is concerned with drive circuits for power switching transistors utilized in power conversion circuits. It is specifically concerned with drive circuitry that is operative to maintain the operating locus of the power switching transistor operating points within safe operating limits and, more particularly, it is concerned with improvements to such circuits to improve the operating locus at certain critical regions to minimize device dissipation thereby improving the conversion efficiency of the power switching circuit.
2. Description of the Prior Art
Continued efficient and reliable operation of high power switching transistors in power conversion circuits, requires a base drive circuit that consistently operates these transistors within safe operating limits defined by the characteristic parameters of the transistor. Consideration of secondary breakdown failure in the transistor, due in part to circuit parameters, is an added reason to strictly maintain its operating locus within strictly defined safe voltage and current limits, since these limits define the power dissipation that can be handled safely by the power transistor. These limits tend to be most critical at certain well-defined transition stages of the operating point locus.
Conventionally, recognized operating stages of power switching transistors usually include a conducting state and a nonconducting state and the intervening turn-on/turn-off switching transition states. During each one of these operating states, the drive circuit must operate to minimize circuit and transistor power dissipation and maintain the operating locus of the power switching transistor within the safe operating region to assure its reliable operation.
For example, in the conducting state, the base drive circuit must provide high current drive levels to bias the transistor deep into saturation in order to keep the voltage drop across the conductive path of the transistor at the lowest possible level to minimize power dissipation within the transistor. Failure to keep this drive high could result in significant power dissipation therein, lowering the conversion efficiency of the switching circuit and in the extreme case, eventually cause a thermal runaway condition leading to the destruction of the transistor.
During the nonconducting state, the drive circuit operates to keep the transistor biased nonconducting. Negligible current flows through the transistor thereby preventing power dissipation in the device.
Normally in power switching transistors, the most critical relationship is power dissipation associated with the switching between states. The transition states from on to off and off to on are critical, due to the undesirable simultaneous occurrence of significant voltage and current levels across and through the main conductive path of the transistor.
For example, during the turn-on transition interval, when the transistor is switching from a nonconducting state to a conducting state, the base drive circuit must drive the transistor through this transition state to the conduction state as quickly as possible in order to reduce the time length interval of simultaneous high voltage and high current in the transistor to as small a value as possible. To achieve this objective, the base drive circuit must supply a large initial base drive current to the transistor.
During the turn-off transition interval, when the transistor is switched from a conducting state to a nonconducting state, the base drive circuit must provide a reverse base drive current to remove excess stored charge in the base-emitter junction to shorten this delay. After this delay time, the circuit applies a reverse bias voltage across the base-emitter junction to counteract the effects of any remaining stored charge in the device to avoid current "tailout" or the asymptotic decrease in collector current to zero with large collector-to-emitter voltage across the transistor. If a turn-on or turn-off operation is too slow, the power dissipation in the transistor could degrade the conversion efficiency of the switching circuit and in the extreme case cause a thermal runaway condition.
One technique in the prior art for achieving some of these objectives has been to use regenerative base drive arrangements in which, inductive coupling of the collector current is used to supply a proportional base drive current to the power transistor. Such an approach is disclosed in U.S. Pat. Nos. 3,999,086; 3,983,418 and 3,930,170. These regenerative arrangements provide the necessary gain to assure that the locus of the operating points of the switching transistor in its conducting states minimize power loss and is safely within a saturated conductive region. However at the initiation of the conduction state during the turn-on transition time, the collector-emitter voltage of the switching transistor does not drop to the low saturated value of the conduction state until a time interval of several microseconds has passed. Since this delay in voltage reduction is coincidental with a high collector current in the transistor, significant power dissipation occurs within the switching transistor. Prior art regenerative circuits do not supply a sufficient base drive current and permit a quasi-saturation region resulting from apparent lower device gain during this interval.
A second problem manifests itself when a power switching transistor is operated at a relatively high frequency, due to stored charge in the junction regions. It is well-known in the art that this charge storage tends to prolong current conduction in the transistor beyond the application of cut-off base drive.
A transistor base drive circuit disclosed in U.S. Pat. No. 3,999,086 has attempted to counteract this stored charge problem by using a transistor switch to supply reverse drive current derived from the regenerative feedback network to assist in removing the stored charge. However, the time required by this transistor switch to fully conduct limits its effectiveness in eliminating the turn-off delay due to stored charge.
To achieve high efficiency in switching high power transistors at a relatively high frequency, the base drive circuitry must function to effectively eliminate the quasi-saturation region and, in addition, speed up the elimination of stored charge therein during the reverse drive mode and, of course, provide sufficient forward drive to maintain the device in "hard" saturation during the conduction interval.